An air conditioning system is typically one of the high power consumption devices in a house or office building. With a traditional air-conditioning system that is controlled by a typical thermostat, the system typically cycles ON and OFF according to the differences of preset and measured temperature. The system draws a constant current or power when it is operating in the ON cycle. It essentially shuts off when in operating in the OFF cycle, and consequently no current or power is drawn from the electrical main circuit when it is operating in the OFF cycle. In order to estimate the energy consumption of the system, one may calculate the accumulated ON time and multiple it by the power consumption of the device to obtain the total energy consumption of the system.
With the above scenario, the energy (power) consumption of a device may be obtained from the specification or by a simple current meter. The same approach applies to week and month estimation of the energy consumption. However, the above approach may no longer hold if the compressor or motor is a variable speed device. For a variable speed compressor, the speed of the compressor may vary according to the differences of the ambient and set temperature. The power consumption varies with the speed of the motor/compressor and is no longer constant. Consequently, calculating the energy consumption of a device simply using ON/OFF duty cycle information from the thermostat is typically not adequate.
With the need to conserve electrical energy (power) usage, it is important for the actual electrical energy of an electrical device to be measured and reported.